Blade for a gas turbine

ABSTRACT

A blade for a gas turbine includes an airfoil extending in a longitudinal direction and extending transversely to the longitudinal direction between a leading edge and a trailing edge. The airfoil has a pressure side, a suction side, and a slot-like cooling medium outlet extending along the trailing edge. The cooling medium outlet is configured to discharge cooling medium supplied from an inner space of the blade. A platform extends transversely to the longitudinal direction. An end of the airfoil merges into an underside of the platform and has a transition from the airfoil to the platform at the trailing edge of the airfoil that increases in thickness in a direction toward the underside of the platform. The cooling medium outlet extends into the platform so as to reduce an operating temperature in a region of the transition from the blade airfoil to the platform.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to International Patent Application No. PCT/EP2009/062213, filed Sep. 21, 2009, which claims priority from Swiss Patent Application No. 01548/08, filed Sep. 30, 2008, each of which are incorporated by reference herein in their entirety. The International Application was published as WO2010/037659 on Apr. 8, 2010.

FIELD

The present invention relates to the field of gas turbines, and particularly relates to a blade for a gas turbine.

BACKGROUND

The requirements for increasing the efficiency of gas turbines leads to the thickness at the trailing edges of the blade airfoils of the blades which are fitted in the gas turbines having to be continuously reduced. This results in a geometry of the blade as is exemplarily shown in cross section in FIG. 1. The blade 10 of FIG. 1 extends in the manner of an airfoil profile transversely to its longitudinal direction between a rounded leading edge 15 and a comparatively sharply tapering trailing edge 16. The blade 10 has a (concave) pressure side 13 and a (convex) suction side 14 with corresponding walls 13′ and 14′. A gaseous cooling medium or coolant is fed in the hollow inner space 17 and discharged into the environment inter alia through a cooling medium outlet which is formed at the trailing edge 16. A particularly sharply tapering, slender trailing edge 16 in this case is achieved by the cooling medium outlet 18 being arranged entirely on the pressure side 13 of the blade 10, and by the two walls 13′ and 14′ being constructed especially thin in the region of the trailing edge 16.

If, as is shown in the perspective view of FIG. 2, the blade airfoil 11 at the end of its extent merges in the longitudinal direction into a platform 12 which lies transversely to the longitudinal direction and is delimited by this platform 12, the transition of the blade airfoil 11 to this platform 12 in the region of the trailing edge 16 represents a typical factor which limits the service life of a gas turbine component because it is exposed to superposition of high thermal stress which is brought about by the thermo-mechanical mismatch between platform 12 and blade airfoil 11, and to mechanical stress peaks which are brought about by loading of the blades as a result of the gas flow. Reducing the thickness of the trailing edge 16 causes an increase of the stress in this critical region so that when designing the blade measures have to be considered in order to achieve and to ensure a sufficiently long service life.

SUMMARY

An aspect of the present invention is to further develop a blade for a gas turbine so that despite a low thickness at the trailing edge of the blade airfoil a satisfactory service life is achieved.

In an embodiment, the present invention provides a blade for a gas turbine including an airfoil extending in a longitudinal direction and extending transversely to the longitudinal direction between a leading edge and a trailing edge. The airfoil has a pressure side, a suction side, and a slot-like cooling medium outlet extending along the trailing edge. The cooling medium outlet is configured to discharge cooling medium supplied from an inner space of the blade. A platform extends transversely to the longitudinal direction. An end of the airfoil merges into an underside of the platform and has a transition from the airfoil to the platform at the trailing edge of the airfoil that increases in thickness in a direction toward the underside of the platform. The cooling medium outlet extends into the platform so as to reduce an operating temperature in a region of the transition from the blade airfoil to the platform.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are described in more detail below, with reference to the drawings. Some non-essential elements of the invention have been omitted. Like elements are provided with the same designations in the different figures. In the drawings:

FIG. 1 shows a simplified cross section through an gas turbine blade with a narrow trailing edge and a cooling medium outlet;

FIG. 2 shows a sharp transition between blade airfoil and platform in a blade such as that shown in FIG. 1; and

FIG. 3 shows a low-stress transition between blade airfoil and platform according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

In an embodiment, the present invention provides a transition from the blade airfoil to the platform at the trailing edge that has a transition thickness profile in which the thickness increases above average the closer it gets to the underside of the platform, and that the cooling medium outlet is extended right into the platform for reducing the temperature in the region of the transition from the blade airfoil to the platform. As a result of increasing the thickness of the trailing edge towards the platform the mechanical stress in the transition region is reliably reduced. Extending the cooling medium outlet right into the platform leads to improved cooling there so that thermally induced stresses are also significantly reduced.

In one embodiment, the transition thickness profile has an essentially exponential shape which resembles an inverted, very slender, truncated pyramid or an inverted virtual pyramid. As a result of this an especially “smooth” transition between trailing edge and platform is achieved.

In another embodiment, the transition from the blade airfoil to the platform has an approximately elliptical transition border profile which also reduces stresses.

Furthermore, it is advantageous if according to another development the trailing edge at the transition from the blade airfoil to the platform is extended up to the edge of the platform.

In an other embodiment, the cooling medium outlet is formed between a pressure-side wall of the blade airfoil and a suction-side wall of the blade airfoil, and in that the pressure-side wall has a curvilinear transition edge profile in the transition from the blade airfoil to the platform in such a way that the wall thickness of the pressure-side wall in the region of the transition from the blade airfoil to the platform is approximately equal to the wall thickness in the remaining region of the blade airfoil.

In FIG. 3, a blade 20 for a gas turbine with a low-stress transition between blade airfoil 11 and platform 12 according to an exemplary embodiment of the invention is reproduced. The blade 20 of the exemplary embodiment comprises a blade airfoil 11 which extends in a longitudinal direction and in the manner of a wing extends transversely to the longitudinal direction between a leading edge 15 and a trailing edge 16, and has a pressure side 13 and a suction side 14. At the upper (or lower) end the blade airfoil 11 merges into a platform 12 which lies transversely to the longitudinal direction and projects laterally across the blade cross section. A slot-like cooling medium outlet 18 which extends along the trailing edge 16 is provided at the trailing edge 16 of the blade airfoil 11, through which a cooling medium, for example cooling air, which is fed via the (hollow) inner space 17 of the blade 20, is discharged. The trailing edge 16 with its thin walls 13′ and 14′ is very narrow in construction. In order to reduce the thermal stresses at the transition between the narrow trailing edge 16 and the solid platform 12, according to embodiments of the invention the transition has a transition thickness profile 21 in which the thickness D increases above average the closer it gets to the underside 12′ of the platform 12. At the same time, the cooling medium outlet 18 is extended (extension 19) right into the platform 12 for reducing the local temperature in the region of the transition from the blade airfoil 11 to the platform 12.

The transition thickness profile 21 has an essentially exponential shape and as a result resembles an inverted rampant pyramid. It is especially favorable in this case with regard to the stress distribution if the transition from the blade airfoil 11 to the platform 12 has an approximately elliptical transition border profile 22. While in the case of the conventional blade according to FIG. 2 the trailing edge 16 of the blade airfoil 11 terminates inside the platform 12 and does not extend as far as the boundary of the platform 12, in the case of the exemplary embodiment of FIG. 3 the trailing edge 16 at the transition from the blade airfoil 11 to the platform 12 is extended up to the edge 12″ of the platform 12.

As is to be seen in FIG. 3, the cooling medium outlet 18 is delimited by the pressure-side wall 13′ and the suction-side wall 14′ of the blade airfoil 11. The pressure-side wall 13′ in this case has a curvilinear transition edge profile 23 in the transition from the blade airfoil 11 to the platform 12 in such a way that the wall thickness of the pressure-side wall 13′ in the region of the transition from the blade airfoil 11 to the platform 12 is approximately equal to the wall thickness in the remaining region of the blade airfoil 11.

In all, an appreciable improvement of the service life at the transition between the blade-airfoil trailing edge and the platform of a gas turbine blade is achieved by the invention as a result of the following measures:

-   -   (1) Extending the cooling medium outlet (cooling slot) into the         platform in order to reduce the metal temperature in the         critical region by feeding cooling medium, wherein convective         cooling of the walls on both sides takes place.     -   (2) Relocating the blade-airfoil trailing edge to the boundary         of the platform in order to lower stress and to make the design         of the blade independent of deviations in the radial position of         the cast core.     -   (3) Introducing a transition thickness profile of a rampant         pyramid type by increasing the height and introducing a special         elliptical contour of the fillet at the transition between blade         airfoil and platform in the region of the trailing edge.     -   (4) Introducing a specially curvilinear transition edge profile         towards the pressure side of the blade airfoil on the fillet at         the transition between blade airfoil and platform in the region         of the trailing edge in order to achieve a wall thickness in the         transition region which corresponds to the wall thickness of the         blade airfoil, as a result of which stress and metal temperature         is reduced and service life at the transition is increased.

List of Designations

10, 20 Blade (gas turbine)

11 Blade airfoil

12 Platform

-   -   12′ Underside (platform)

12″ Edge (platform)

13 Pressure side

13′ Wall (pressure side)

14 Suction side

14′ Wall (suction side)

15 Leading edge

16 Trailing edge

17 Inner space

18 Cooling medium outlet (slot-like)

19 Extension (cooling medium outlet)

21 Transition thickness profile

22 Transition border profile

23 Transition edge profile

D Thickness (transition thickness profile) 

1. A blade for a gas turbine, the blade comprising: an airfoil extending in a longitudinal direction and extending transversely to the longitudinal direction between a leading edge and a trailing edge, the airfoil having a pressure side, a suction side, and a slot-like cooling medium outlet extending along the trailing edge, the outlet configured to discharge cooling medium supplied from an inner space of the blade; a platform extending transversely to the longitudinal direction, a first end of the airfoil merging into an underside of the platform and having a transition from the airfoil to the platform at the trailing edge of the airfoil that increases in thickness in a direction toward the underside of the platform, and wherein the cooling medium outlet extends into the platform so as to reduce an operating temperature in a region of the transition from the blade airfoil to the platform.
 2. The blade as recited in claim 1, wherein the transition from the blade airfoil to the platform has a thickness profile having a substantially exponential shape corresponding to at least one of an inverted rampant pyramid, an inverted truncated pyramid, and an inverted virtual pyramid.
 3. The blade as recited in claim 1, wherein the transition from the blade airfoil to the platform has an approximately elliptical profile.
 4. The blade as recited in claim 1, wherein the transition from the blade airfoil to the platform extends to an edge of the platform.
 5. The blade as recited in claim 2, wherein the transition from the blade airfoil to the platform extends to an edge of the platform.
 6. The blade as recited in claim 3, wherein the transition from the blade airfoil to the platform extends to an edge of the platform.
 7. The blade as recited in claim 1, wherein the cooling medium outlet is formed between a pressure-side wall of the blade airfoil and a suction-side wall of the blade airfoil, and wherein the transition from the blade airfoil to the platform along the pressure-side wall has a curvilinear edge profile such that a wall thickness of the pressure-side wall in a region of the transition is approximately equal to a wall thickness in a remaining region of the blade airfoil.
 8. The blade as recited in claim 7, wherein the transition from the blade airfoil to the platform extends to an edge of the platform. 